Latent curing agent and epoxy compositions containing the same

ABSTRACT

A latent curing agent obtained by the reaction of a coumarin compound with an amine.

The present invention is directed to a latent curing agent, and curablecompositions containing said latent curing agent and at least onepolyepoxide, a method for the preparation of said curing agent, and theuse of the curable composition comprising said latent curing agent and apolyepoxide.

Numerous compositions and processes are described in the art for makingand using a wide variety of epoxy-based compositions and additives in aneffort to improve the curing properties, work-life and or adhesivestrength and other key properties of adhesives useful in adhering,filling and making composite structures.

Epoxy-amine curing systems are one of the most widely appliedformulations to adhesives, sealants, and coatings. The curing reaction,comprising an addition reaction between the amine and epoxide groups,usually occurs spontaneously at ambient temperature. To avoid theepoxy-amine reaction, epoxy resin and amine hardener have to be mixedimmediately before the application of the curable composition. Yetcuring reaction of such a system does not start before mixing, themixing process immediately stimulates viscosity to increase, thusprohibiting easy mixing and application. In order to improve handlingproperties of these epoxy compositions it is desirable to give latentproperties to the epoxy-amine curing systems, so that curing occurs onlyat elevated temperatures.

One component systems (frequently also referred to as “one packagesystems”) are highly desirable because they allow manufacturers andconsumers of epoxy compositions to avoid more complex packaging means,and to avoid the additional mixing step necessary before a separatelypackaged epoxy resin and curative can be converted into a curedmaterial, and to avoid the probability that an incorrect amount ofcuring agent will be added to the resin by the ultimate consumer.

One of the oldest latent curatives is Dicyandiamide. It is oftenconsidered to be the “workhorse” of all one component (1K) epoxy curingagents due to its ease of use, excellent performance properties, longshelf stability and low toxicity. Dicyandiamide is supplied in a whitecrystalline powder form, which is only sparsely soluble in liquid epoxyresins. While Dicyandiamide performance properties in epoxy adhesivesare in high demand, at times its activation temperature is too high forthe desired efficiency in application and curing performance.

To improve the latent properties of amine hardeners, they are usuallychosen from those which are solid at ambient temperatures to be able tocoexist with epoxy compounds in a one package system for a reasonableperiod of time. The curing reactions of such compositions start at anelevated temperature which causes melting of the hardeners to allow thecuring reaction with the epoxides. However, choices of such latent aminecompounds are limited.

U.S. Pat. No. 3,488,742 and U.S. Pat. No. 3,639,657 disclose thereaction products of approximately equimolecular proportions of an acidanhydride, such as phthalic anhydride, and a polyamine, such asdiethyl-enetriamine. They are reported to be effective latentaccelerators for dicyandiamide in the curing of epoxy resin systems. Thecombination of dicyandiamide with the said reaction product of the acidanhydride and polyamine are said to provide systems which, when combinedwith epoxy resins, will be stable for long periods when stored atambient temperatures while still providing hardened products ofsatisfactory or good properties on curing for relatively short periodsat elevated temperatures, in the order of 100° C. to 150° C.

According to EP 440583 reaction products of polyallylnadic anhydride andvarious polyamines, such as ethylene diamine, diethylene triamine,triethylene tetramine, 1,3-diaminopropane, 1,6-diaminohexane,imino-bis(propyl amine) and methyl-imino-bis(propylamine), are latenthardeners for use in epoxy resins having more than one 1,2-epoxy groupsper molecule. Mixtures of these products and epoxy resins are said to bestable at room temperature for extended periods, yet cure quickly atelevated temperatures to yield products with high glass transitiontemperatures.

From U.S. Pat. No. 3,261,882 adhesives are known, which are made frompolyisophthalamides and thermosetting epoxy resin syrups. According tothis reference, the polyisophthalamides can be made among others fromC₂- to Ci₆-diamines, such as ethylenediamine, propylenediamine orhexamethylenediamine, and isophthalic acid.

Reaction products of a carboxylic acid-containing microgel, especially acrosslinked copolymer, and a nitrogen containing base are disclosed inEP816393. Said microgels can be prepared by reaction of a carboxylicacid with an N-containing base at 0° C.-200° C., wherein the number ofcarboxylic acid groups is at least equal to the number of basic N atoms.Also disclosed are epoxy resin compositions containing the reactionproduct as a hardener. According to this reference, it is preferred thatthe reaction product is a copolymer of an unsaturated carboxylic acidwith a multifunctional crosslinker and optionally also a COOH group-freevinyl monomer. As suitable unsaturated acids are mentioned (meth)acrylicacid, 2-carboxyethyl (meth)acrylate, phthalic acid (2-(meth)acryloyl)ester, maleic- or fumaric-acid, (or their mono-methyl- or-ethyl-esters), itaconic acid, cinnamic acid, crotonic acid,4-vinylcyclohexane carboxylic acid, 4-vinylphenylacetic acid or4-vinylbenzoic acid. The crosslinker is especially ethylene- orpropylene-glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate,polyethylene- or polypropylene-glycol di(meth)acrylate,1,1,1-trimethylolpropane tri(meth)acrylate, bisphenol-A diglycidyletherdi(meth)acrylate, (meth)acrylic acid allyl ester, divinylcyclohexane ordivinylbenzene. The non-carboxylic vinyl monomer is Me-, Et- orBu-(meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-ethylhexyl(meth)acrylate or styrene. As suitable nitrogen base are disclosed anamine, poly-amine or imidazole.

Thus in the art a variety of amine hardeners are modified with othercompounds to reduce activity of amino groups and/or to convert them intoa solid state at ambient temperature.

There is still a need for amine hardeners with effective deactivation ofprimary amino groups towards epoxides to result in improvement oflatency of the curable composition. In addition an improvement ofadhesion to metal surfaces is desirable, as well as a reduction of theheat required for curing the composition. Moreover an acceleration ofthe curing reaction of an epoxy-imidazole curing system under anelevated temperature would be advantageous.

One object of the present invention is to provide a latent curing agentobtained by the reaction of a coumarin compound of the general formula(1)

with an amine of the general formula (2)

(H₂N-A)_(a)-B  (2)

In formula (1) X and Y can be independently H, an alkyl group, aheteroalkyl group, an aromatic group, a heteroaromatic group or anacetyl group, and Z can be H, OH, an alkyl group, a heteroalkyl group,an aromatic group, a heteroaromatic group.

In formula (2), “a” is an integer of 1 or 2, A can be an alkylene group,a heteroalkylene group, an aromatic group, a heteroaromatic group, and Bcan be an alkyl group (if a=1), an alkylene group (if a=2), aheteroalkyl group (if a=1), a heteroalkylene group (if a=2), an aromaticgroup, a heteroaromatic group, a hydroxyl group (if a=1), a secondaryamino group (if a=2), O or S (if a=2), whereby B has the valency “a”(meaning that B forms a number of “a” bonds to A), or where B alone orwhere A and B together form a ring system selected from an aliphaticring system, a heteroaliphatic ring system, or an aromatic ring system.A and B can independently be completely saturated (i.e. do not containdouble or triple bonds), partially unsaturated (i.e. contain one or moredouble or triple bonds), or contain or form aromatic ring systems.

For example, A in formula 2 can be a —(CH₂—)_(x) group with, forexample, x=1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 ore more. In this case,if “a”=1, B may be a methyl group (so that the amine is amonoalkylamine), a hydroxyl group (so that the amine is amonoalkanolamine), or “a”=2 and B is a methylene group, so that either alinear alkylene diamine is formed, or A and B together form a ringsystem. If “a”=2 and B is —O— or —S—, the amine is a di(aminoalkyl)etheror -thioether. Alternatively, A and B may be selected in a way that theyform together a poly(oxyalkylene) chain, so that the amine is apoly(oxyalkylene)diamine, e.g. of the type known as a “Jeffamine™”.Furthermore, A and B may be selected in a way that the amines explicitlynamed below are formed.

A further embodiment of the present invention is a curable compositioncontaining at least one latent curing agent as identified above and apolyepoxide which has at least 2 1,2-epoxide groups per molecule.

Still an other object of the present invention is a method of making acuring agent from a cou-marin compound of formula (1) and an amine offormula (2) comprising the steps of

(a) mixing at least one of the mono- or diamines with the coumarin,optionally in the presence of an polar solvent,

(b) heating the reaction mixture to a temperature between roomtemperature (20° C.) and 70° C. until the monoamine or diamine hascompletely reacted with the coumarin,

(c) precipitating the reaction mixture into a nonsolvent for thecoumarin amine reaction product,

(d) filtering the precipitate from the liquid phase, and

(e) drying the coumarin amine reaction product.

Examples of suitable coumarin compounds of formula (1) are7,8-dihydroxy-1-benzopyran-2-one (also known as daphnetin),6,7-dihydroxy-1-benzopyran-2-one (also known as esculetin),7-hydroxy-1-benzopyran-2-one (also known as umbelliferone),4-methyl-7,8-dihydroxy-1-benzopyran-2-one (also known as4-methyldaphnetin) and especially the basic 1-benzopyran-2-one(coumarin).

The amine of formula (2) can preferably be selected from monoamines asexemplified by a group consisting of butylamine, pentylamine,hexylamine, heptylamine, octylamine, nonylamine, decylamine,dodecylamine, cyclohexylamine, aminomethylcyclohexane,N-aminoethyl-piperidine, 1-amino-3,5,5-trimethyl-cyclohexane,benzylamine, aminophenol, 2-aminoethanol, 3-amino-1-propanol,4-amino-1-butanol, 5-amino-1-pentanol, or polyamines, particularlydiamines as exemplified by the group consisting oftetramethylenediamine, hexamethylenediamine (HMDA),2-methylpentamethylenediamine, nonamethylenediamine,undecamethylenediamine, dodecamethylenediamine,2,2,4-trimethylhexamethylenediamine,2,4,4-trimethylhexa-methylenediamine, 5-methyl-nonamethylenediamine,1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)-cyclohexane,1-amino-3-aminomethyl-3,5,5-trimethyl-cyclohexane,bis(4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane,2,2-bis(4-aminocyclohexyl)-propane, polyoxytetramethylenediamine,triethyleneglycoldiamine, poly-oxyethylenediamines,polyoxyethylenetriamines, polyoxypropylenediamines (also known under thetrade name “Jeffamine™” from Huntsman), m-xylylenediamine (m-XDA),p-xylylenediamine (p-XDA), 1,4-bis(aminopropyl)piperidine (BAPP),1-propanamine, 3,3′-(oxy bis(2,1-ethanediyloxy))bis-1-propan-amine,(diaminopropylated diethylene glycol, also known under the trade name“ANCAMINE™ 1922A” from Air Products), 1,3-diaminocyclohexane,1,4-diaminocyclohexane, di(aminomethyl)cyclohexane,1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane (commonly called“isophoronediamine”), cyclohexylenediamine,4,4′-isopropylidenedicyclohexyldiamine, and3,3′-dimethyl-4,4′-isopropylidenedicyclohexyldiamine, or mixtures of themono- and/or diamines listed above.

The reaction products of coumarin with monoamines can be quantitativelyobtained from the precursor monoamine compounds in one step, by thereaction of the monoamine with the selected coumarin. It may beappropriate to use a polar solvent such as methanol, ethanol, acetone,dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA) ortetrahydrofurane (THF). If necessary, mixtures of polar solvents can beused. The reaction product is normally obtained in quantitative yieldand has the following general formula (3).

In formula (3), A, B and Z have the same meaning as defined above for“a”=1.

Typically, the primary monoamine is reacted at a stoichiometric ratio of2 equivalents of amine to 1 equivalent of the coumarin.

If desired, the reaction rate between coumarin and the amine can beaccelerated by adding basic catalysts to the reaction mixture. Suitablebasic catalysts are tertiary amines, such as trimethylamine,triethylamine, tributylamine, N,N-dimethylaniline,N,N-dimethyl-benzylamine, pyridine, N-methylpiperidine,N-methylmorpholine, N,N-dimethylaminopyridine, derivatives of morpholinesuch asbis(2-(2,6-dimethyl-4-morpholino)ethyl)-(2-(4-morpholino)ethyl)amine,bis(2-(2,6-dimethyl-4-morpholino)ethyl)-(2-(2,6-diethyl-4-morpholino)ethyl)amine,tris(2-(4-morpholino)ethyl)amine, tris(2-(4-morpholino)propyl)amine,guanidines, such as 1,1,3,3-tetramethyl guanidine, diazabicyclooctane(DABCO), and especially heterocyclic compounds having an amidine bondingsuch as diazabicyclononene (DBN) or diazabicycloundecene (DBU).

If the selected amine is a primary diamine, it is reacted at astoichiometric ratio of 1 equivalent of amine to 1 equivalent of thecoumarin basically under similar reaction conditions as the reaction ofthe monoamine, including the optional use of solvents and/or catalysts.The separation of the reaction products from the synthesis mixture canbe done by precipitation into non-polar solvents which are non solventsfor the adduct. Non solvents or poor solvents for the adduct are forexample diethylether, dichloromethane, alcohols, acetone or mixturesthereof.

In this case, the formed adducts are oligomeric compositions which havethe recurrent structure of formula (4):

In formula (4), A, B and Z have the same meaning as defined above forformulas (1) and (2), respectively, for “a”=2.

Typically, the molecular weight (M_(n)) of the oligomers of formula (4)is in the range between 2000 and 20 000, preferably between 4000 and 10000.

Curable compositions can be formulated by mixing the latent curing agentaccording to formula (3) and/or (4) with at least one polyepoxide whichhas at least 2 1,2-epoxide groups per molecule at room temperature(approximately 20° C.) or slightly elevated temperatures (e.g. up toabout 50° C.).

At least one polyepoxide used may include multifunctionalepoxy-containing components, such as Ci—C₂8 alkyl-, poly-phenol glycidylethers; polyglycidyl ethers of pyrocatechol, resorcinol, hy-droquinone,4,4′-dihydroxydiphenyl methane (or bisphenol F, such as RE-303-S orRE-404-S available commercially from Nippon Kayuku, Japan),4,4′-dihydroxy-3,3′-dimethyldiphenyl methane, 4,4′-dihydroxydiphenyldimethyl methane. (or bisphenol A), 4,4′-dihydroxydiphenyl methylmethane, 4,4′-dihydroxydiphenyl cyclohexane,4,4′-dihydroxy-3,3′-dimethyldiphenyl propane, 4,4′-dihydroxydiphenylsulfone, and tris(4-hydroxyphenyl)methane; polyglycidyl ethers oftransition metal complexes; chlorination and bromination products of theabove-mentioned diphenols; polyglycidyl ethers of novolacs; polyglycidylethers of diphenols obtained by esterifying ethers of diphenols obtainedby esterifying salts of an aromatic hydrocarboxylic acid with adihaloalkane or dihalogen dialkyl ether; polyglycidyl ethers ofpolyphenols obtained by condensing phenols and long-chain halogenparaffins containing at least two halogen atoms; phenol novolac epoxy;cresol novolac epoxy; and combinations thereof.

Among the commercially available epoxy components suitable for use inthe present invention are polyglycidyl derivatives of phenoliccompounds, such as those available under the tradenames EPON 825, EPON826, EPON 828, EPON 1001, EPON 1007 and EPON 1009, cycloaliphaticepoxy-containing compounds such as Araldite CY179 from Huntsman orwater-borne dispersions under the tradenames EPI-REZ 3510, EPI-REZ 3515,EPI-REZ 3520, EPI-REZ 3522, EPI-REZ 3540 or EPI-REZ 3546 from Hexion;DER 331, DER 332, DER 383, DER 354, and DER 542 from Dow Chemical Co.;GY285 from Huntsman, Inc.; and BREN-S from Nippon Kayaku, Japan. Othersuitable epoxy-components include polyepoxides prepared from polyols andthe like and polyglycidyl derivatives of phenol-formaldehyde novolacs,the latter of which are available commercially under the tradenames DEN431, DEN 438, and DEN 439 from Dow Chemical Company and a waterbornedispersion ARALDITE PZ 323 from Huntsman.

Cresol analogs are also available commercially such as ECN 1273, ECN1280, ECN 1285, and ECN 1299 or waterborne dispersions ARALDITE ECN 1400from Huntsman, Inc. SU-8 and EPI-REZ 5003 are bisphenol A-type epoxynovolacs available from Hexion. Epoxy or phenoxy functional modifiers toimprove adhesion, flexibility and toughness, such as the HELOXY brandepoxy modifiers 67, 71, 84, and 505. When used, the epoxy or phenoxyfunctional modifiers may be used in an amount of about 1:1 to about 5:1with regard to the heat curable resin.

Of course, combinations of the different epoxy resins (epoxy components)are also desirable for use herein.

The curing of the epoxy resin(s) utilized in the present invention maybe additionally assisted by the incorporation of other substancescapable of promoting the desired hardening upon heating. Such materialsare referred to herein as “curatives”, but also include the materialsreferred to by workers in the field as curing agents, hardeners,accelerators, activators, and catalysts. While certain curatives promoteepoxy resin curing by catalytic action, others participate directly inthe reaction of the resin and become incorporated into the thermosetpolymeric network which is formed. Although any of the curatives (curingagents) known in the epoxy resin field (see the corresponding chapter inthe Encyclopedia of Polymer Science and Engineering) may be used in thepresent invention in addition to the amine-epoxy adduct, the use of oneor more nitrogen-containing compounds such as imidazoles—including forexample 2-methyl imidazole, 2,4-dimethyl imidazole, 2-ethyl-4-methylimidazole, 2-phenyl imidazole, 1-benzyl-2-methylimidazole (BMI) and thelike—, heterocyclic compounds having an amidine bonding such asdiazabicyclo-nonene (DBN) or diazabicycloundecene (DBU), substitutedureas such as p-chlorophenyl-N,N-dimethylurea (MONURON),3-phenyl-1,1-dimethylurea (FENURON) or3,4-dichlorophenyl-N,N-dimethylurea (DIURON), amino compounds liketertiary aryl- or alkylamines, such as, for example,benzyldimethylamine, tris(dimethylamino)phenol, piperidine or piperidinederivatives, amine salts, and quaternary ammonium compounds as theauxiliary curative(s) is desirable (provided that such compounds whichcause an unacceptably high degree of epoxy resin reaction under normalstorage conditions are avoided). Dicyandiamide (sold commercially by AirProducts under the trademark “AMICURE CG”) is a particularly preferredauxiliary curative, although other guanidine compounds may also beutilized. In one desirable embodiment of the invention, di-cyandiamide(preferably, about 0.5-8 wt. % based on the total weight of thethermosettable composition) is used in combination with an amine-epoxyadduct (preferably, about 0.1-5 wt. %) in the curative system. Thecurative system must, however, be selected such that it does notcatalyze curing of the thermosettable composition to any significantextent under typical storage conditions over an extended period.Preferably, the amounts and identities of the components of the curativesystem are selected such that the thermosettable composition remainsstable for at least two weeks in storage at about 50° C. without asignificant loss in cured properties, but cures within about 10 minutesupon being heated at about 135° C. Particularly preferred curatives areheterocyclic amidines such as DBU, DBN, imidazole and its derivatives.

Other additives which the inventive curable compositions can include aretougheners, plasticizers, extenders, reactive diluents, microspheres,fillers, and reinforcing agents, for example coal tar, bitumen, textilefibres, glass fibres, asbestos fibres, boron fibres, carbon fibres,mineral silicates, mica, powdered quartz, hydrated aluminum oxide,bentonite, wollastonite, kaolin, silica, aerogel or metal powders, forexample aluminium powder or iron powder, and also pigments and dyes,such as carbon black, oxide colors and titanium dioxide, fire-retardingagents, thixotropic agents, flow control agents, such as silicones,waxes and stearates, which can, in part, also be used as mold releaseagents, adhesion promoters, antioxidants and light stabilizers, theparticle size and distribution of many of which may be controlled tovary the physical properties and performance of the inventivepolymerizable composition.

When used, fillers are used in an amount sufficient to provide thedesired rheological properties. Fillers may be used in an amount up toabout 50 percent by weight, such as about 5 to about 32 percent byweight, for instance about 10 to about 25 percent by weight, relative tothe total weight of the composition. The fillers may be inorganic ones,such as silicas. For instance, the silica filler may be a silicananoparticle.

Reactive diluents are another constituent of the binder compositionaccording to the one embodiment of the invention. Reactive diluents inthe context of this invention are low-viscosity substances which containepoxide groups (glycidyl ethers or glycidyl esters) and have analiphatic or aromatic structure. These reactive diluents on the one handserve to lower the viscosity of the binder system above the softeningpoint, and on the other hand they control the pre-gelling process ininjection moulding. Typical examples of reactive diluents which can beemployed according to the invention are mono-, di- or triglycidyl ethersof C6- to C14-monoalcohols or alkylphenols and the monoglycidyl ethersof cashew nut shell oil, diglycidyl ethers of ethylene glycol,diethylene glycol, triethylene glycol, tetraethylene glycol,1,2-propylene glycol, 1,4-butylene glycol, 1,5-pentanediol,1,6-hexanediol or cyclohexanedimethanol, triglycidyl ethers oftrimethylolpropane and the glycidyl esters of C6- to C₂₋₄-carboxylicacids or mixtures thereof.

Optionally, at least one diluent is a heat transfer fluid; morepreferably a heat transfer fluid derived from an aromatic oil, a glycoloil, a petroleum oil, a fluorocarbon oil, and/or a silicone oil; andmost preferably a heat transfer fluid with oxidative stability.Preferred heat transfer fluids include MARLOTHERM LH, MARLOTHERM N,MARLOTHERM P1, MARLOTHERM P2, MAR-LOTHERM SH, and MARLOTHERM X fromSasol North America Inc.

In order to obtain expandable structural foams the composition willadditionally comprise a “blowing agent”. All known blowing agents, suchas e.g. the “chemical blowing agents” which liberate gases bydecomposition or “physical blowing agents”, i.e. expanding hollow beads,are in principle suitable as the blowing agent. Examples of physicalblowing agents are expandable microspheres which are available from AkzoNobel AB under the trademark EXPANCEL or from Henkel under the trademarkDUALITE. Examples of chemical blowing agents areazobisisobu-tyronitrile, azodicarboxamide,di-nitroso-pentamethylenetetramine, 4,4′-oxybis(benzenesulfonic acidhydrazide), diphenyl-sulfone-3,3′-disulfohydrazide,benzene-1,3-disulfohydrazide and p-toluenesulfonyl semicarbazide.

Preferred curable compositions of the present invention will comprise

(a) at least one reactive epoxy resin 10-60 wt. %, preferably 30-40 wt.%,

(b) at least one latent curing agent according to this invention 2-50wt. %, preferably 10-40 wt. %,

(c) optionally a flexibilizing agent 0-25 wt. %, preferably 1-15 wt. %,

(d) optionally a reactive diluent 0-15 wt. %, preferably 1-10 wt. %,

(e) optionally least one finely divided filler 0-50 wt. %, preferably5-30 wt. %,

(f) optionally an accelerator 0-5 wt. %,

(g) optionally a blowing agent. 0-3 wt. %,

the sum of the total constituents being 100 wt %.

To prepare the curable compositions of the present invention, the epoxyresin or resins, the latent curing agent and the optional componentssuch as the flexibilizing agent, the reactive diluent together with thefillers, fibers and pigments are homogenized in a conventional mixingunit, such as a planetary mixer, kneader, speed mixer or the like. Atthe last stage the optional accelerator and/or blowing agents will beblended in. Care should be taken, that the temperature of the mass doesnot exceed about 40° C. to 60° C. during mixing.

As noted, the curable compositions according to this invention are inparticular suitable as coatings, adhesives—particularly structuraladhesives-, sealants, encapsulants, reinforcing structural foams andmatrices for the preparation of reinforced materials such as prepregsand tow-pregs, and/or can be used in injection molding or extrusion orin the formation of prepregs or towpregs formed from a layer or bundleof fibers infused with the polymerizable composition. The curablecompositions are basically thermo-setting, 1K or 2K componentsepoxy-amine compositions.

The invention is illustrated in more detail in the following embodimentexamples, where the choice of examples is not intended to represent alimitation of the scope of the subject matter of the invention, and ismerely intended to represent individual embodiments and advantageouseffects of the invention by way of a model.

Unless stated otherwise, all the amounts of constituents of thecompositions given in the following examples are parts by weight orpercentage by weight.

EXAMPLES Example 1 Synthesis of a Monoamine Adduct of Coumarin

Coumarin (0.3775 g, 2.583 mmol), 2.0 equimolar of n-hexylamine (0.5324g, 5.261 mmol) and 0.1 equimolar of DBU (0.0418 g, 0.275 mmol) wereadded and mixed in bulk. The mixture was reacted in an oven and kept at50° C. for 10 days. The resultant crude mixture was separated bypreparative Thin Layer Chromatography (TLC)(eluent:ethylacetate/n-hexane=1/4 (vol./vol.)) to give correspondingcoumarin monoamine (1:2) adduct (0.8287 g, 2.378 mmol, yield=92%).

Examples 2-5 Synthesis of a Diamine Adduct of Coumarin

To a DMSO solution of coumarin (5.537 g, 37.89 mmol, concentration=3 M)in a flask, 1.0 equimolar of m-xylylenediamine (m-XDA) (5.168 g, 37.95mmol) and 0.1 equimolar of DBU (0.579 g, 3.803 mmol) were added. Theresultant mixture was reacted at 50° C. for 10 days with stirring. Afterseparation by precipitation into diethylether, the correspondingcoumarin-m-XDA (1:1) adduct was obtained as a precipitation (10.213 g,yield=95%).

Other coumarin-diamine (1:1) adducts were similarly obtained (see Table1).

TABLE 1 Reaction Example Diamine Condition Mn¹⁾²⁾ Mw/Mn^(1),2))Yield^(3),) 2 m-XDA 50° C., 10 days 8400 2.6 95 3 HMDA 50° C., 10 days7600 2.9 100 4 Ancamine 50° C., 10 days 6400 3.2 100 1922A 5 BAPP 60°C., 10 days 4100 3.0 75 Remarks ¹⁾determined by Gel PermeationChromatography (GPC) ²⁾dimethylformamide (DMF) soluble part ³⁾isolatedyield

Examples 6-10 Curable Compositions Containing Amine Adducts of Coumarin

Bisphenol A diglycidyl ether (Bis A-DGE), coumarin amine adduct and,optionally, 1-benzyl-2-methylimidazole (BMI) were mixed with a speedmixer (AR-100, THINKY Corp., Japan) at room temperature in air anddegassed under vacuum to obtain curable compositions according to theinvention listed in table 2.

TABLE 2 Curing agent from example Example epoxide (amount) (amount)accelerator 6 Bis A-DGE (0.900 g) 2 (0.498 g) — 7 Bis A-DGE (2.964 g) 3(1.523 g) — 8 Bis A-DGE (1.456 g) 4 (1.045 g) — 9 Bis A-DGE (2.070 g) 5(1.400 g) — 10 Bis A-DGE (2.569 g) 1 (0.526 g) BMI (0.039 g)

Reference Examples 11-15 Curable Compositions Containing Free Amines

For comparison, curable compositions from Bisphenol A diglycidyl ether(Bis A-DGE) and free amines were mixed with a speed mixer (AR-100,THINKY Corp., Japan) at room temperature in air and degassed undervacuum to obtain reference compositions listed in table 3.

TABLE 3 Curing agent Example Epoxide (amount) (amount) accelerator 11Bis A-DGE (1.096 g) m-XDA (0.219 g) — 12 Bis A-DGE (0.952 g) HMDA (0.162g) — 13 Bis A-DGE (3.200 g) Ancamine 1922A — (1.036 g) 14 Bis A-DGE(2.332 g) BAPP (0.686 g) — 15 Bis A-DGE (1.934 g) Hexylamine BMI (0.029g) (0.115 g.)

Curing Reactions

The curable compositions of examples 6 to 15 were analysed byDifferential Scanning Calo-rimetry (DSC) in a dynamic heating mode (10°C./min). 10 mg of each of the formulations was heated for the DSC. Theresulting heat evolution profiles are shown in table 4. In all cases,utilising the adducts of the invention as hardeners resulted inincreases in onset and peak top temperatures (Table 4), suggesting thatthe inventive compositions 6-10 are much more stable than the referencecompositions 1 1-15. At the same time, the cure heat (ΔH) of thecompositions cured with the inventive amine adducts were much lower thanthose cured with pure (poly)amines, indicating that the cure heat can besuppressed by the use of the present cou-marin-amine adducts ashardeners.

TABLE 4 onset temperature peak temperature Example (° C.) (° C.) Δ H(J/g) 6 108.1 138.1 −215 11 (reference) 73.2 106.1 −369 7 106.2 145.1−241 12 (reference) 72.6 102.5 −409 8 115.5 151.3 −220 13 (reference)65.0 102.7 −326 9 101.8 126.9 −250 14 (reference) 68.3 103.7 −391 10 129.3 136.1 −170 15 (reference) 59.7 123.7 −173

Analysis of the Curable Compositions by DSC in a Isothermally HeatingMode

The compositions were isothermally heated at 50° C. or 100° C. Onlylittle amounts of cure heats were observed, indicating that thereactions were successfully suppressed under these heating conditions.On the other hand, when the temperature was elevated to 150° C., heatevolution due to progress of the curing reaction became apparent andwere brought to the end within 10 min., indicating that the compositionscured fast under this curing condition. In summary, the DSC analyses ofthe inventive compositions revealed that the application of the amineadducts as hardeners allows much higher stability of the compositionsbelow 50° C. than that of the compositions containing the unmodifiedamines as hardeners.

Adhesive Properties

Compositions of example 8 and 10 were tested as adhesives for metalbonding. The adhesion tests were performed according to ASTM D1002method (“Apparent Shear Strength of Single-Lap-Joint Adhesively BondedMetal Specimens by Tension Loading (Metal-to-Metal)”) using a tensiontesting machine (model: RTC-1350A, manufactured by Orientec Corp.,Japan) and as the test specimens (dimension=25.4 by 101.6 by 1.6 mm,grid blasted), mild steel (JISC3141, SPCC-SD), copper (JISH3100, C1100P) and aluminum (JISH4000, A5052P) substrates were utilized. Thetests were performed at room temperature under an atmosphere of 50%relative humidity.

Composition of example 8 and 10 were applied on metal specimens and werecured at 150° C. for 1 h. For three different substrates, thecomposition containing the coumarin-diamine adduct showed higheradhesion strengths than that the composition 13 containing thecorresponding unmodified amine. Likewise composition 10 was employed asadhesive for metal bonding. It was applied on metal specimens and wascured at 150° C. for 1 h. The composition containing the coumarinmonoamine adduct showed higher adhesion strengths than that of thecomposition containing the corresponding unmodified amine (Table 5).

TABLE 5 Lap-shear adhesion strength (MPa) Substrate Compostions of Test1 Test 2 Test 3 Test 4 Test 5 Test 6 Average Steel Example 8 23.3 26.023.3 24.3 26.0 26.7 24.9 Example 13 22.2 16.7 17.8 21.6 21.8 17.7 19.7(reference) Example 10 24.0 24.0 24.0 23.0 24.0 23.0 23.7 Example 1515.6 16.6 13.9 14.6 14.4 — 15.0 Aluminui Example 8 16.9 17.0 17.6 19.118.1 17.5 17.7 11 Example 13 12.8 11.6 11.6 8.8 9.8 — 10.2 (reference)Copper Example 8 18.0 17.4 17.4 20.6 18.0 20.6 19.2 Example 13 10.2 8.48.4 12.0 8.0 — 10.0 (reference)

1. A latent curing agent obtained by the reaction of a coumarin compoundof the general formula (1)

wherein X and Y can be independently H, an alkyl group, a heteroalkylgroup, an aromatic group, a heteroaromatic group or an acetyl group, andZ can be H, OH, an alkyl group, a heteroalkyl group, an aromatic group,a heteroaromatic group, with an amine of the general formula (2)(H₂N-A)_(a)-B  (2) wherein “a” is 1 or 2, A can be an alkylene group, aheteroalkylene group, an aromatic group, a heteroaromatic group, and Bcan be an alkyl group (if a=1), an alkylene group (if a=2), aheteroalkyl group (if a=1), a heteroalkylene group (if a=2), an aromaticgroup, a heteroaromatic group, a hydroxyl group (if a=1), a secondaryamino group (if a=2), O or S (if a=2), whereby B has the valency “a”, orwherein B alone or where A and B together form a ring system selectedfrom an aliphatic ring system, a heteroaliphatic ring system, or anaromatic ring system.
 2. A latent curing agent according to claim 1wherein the amine is a primary monoamine which is reacted at astoichiometric ratio of 2 equivalents of amine to 1 equivalent of thecoumarin.
 3. A latent curing agent according to claim 1 wherein theamine is a primary diamine which is reacted at a stoichiometric ratio of1 equivalents of amine to 1 equivalent of the coumarin.
 4. A latentcuring agent according to claim 3 wherein the diamine is selected fromthe group consisting of tetramethylenediamine, hexamethylenediamine,2-methylpentamethylene-diamine, nonamethylenediamine,undecamethylenediamine, dodecamethylenediamine,2,2,4-trimethylhexamethylenediamine,2,4,4-trimethylhexamethylenediamine, 5-methyl-nonamethylenediamine,1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)-cyclohexane,1-amino-3-aminomethyl-3,5,5-trimethyl-cyclohexane,bis(4-aminocyclo-hexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane,2,2-bis(4-aminocyclohexyl)-propane, polyoxytetramethylenediamine,triethyleneglycoldiamine, polyoxyethylene-diamines,polyoxyethylenetriamines, polyoxypropylenediamines, m-xylylenediamine,p-xylylenediamine (XDA), 1,4-bis(aminopropyl)piperidine, 1-propanamine,3,3′-(oxy bis(2,1-ethanediyloxy))bis-1-propan-amine, diaminopropylateddiethylene glycol, 1,3-diaminocyclohexane, 1,4-diaminocyclohexane,di(aminomethyl)cyclohexane,1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane,4,4′-diamino-3,3′-dimethyldicyclohexylmethane, cyclohexylenediamine,4,4′-diamino-dicyclohexylmethane,4,4′-isopropylidenedicyclohexyldiamine, and3,3′-dimethyl-4,4′-isopropylidenedicyclohexyldiamine or mixturesthereof.
 5. A latent curing agent according to claim 2 where themonoamine is selected from the group consisting of butylamine,pentylamine, hexylamine, heptylamine, octylamine, nonylamine,decylamine, dodecylamine, cyclohexylamine, aminomethylcyclohexane,N-aminoethyl-piperidine, 1-amino-3,5,5-trimethyl-cyclohexane,benzylamine, aminophenol, 2-aminoethanol, 3-amino-1-propanol,4-amino-1-butanol, 5-amino-1-pentanol or mixtures thereof.
 6. A latentcuring agent according to claim 1 wherein the coumarin is1-benzopyran-2-on.
 7. A curable composition comprising (a) at least onereactive epoxy resin, (b) optionally a flexibilizing agent, (c)optionally a reactive diluent, (d) at least one finely divided filler,(e) a latent curing agent according to claim 1, (f) optionally anaccelerator, (g) optionally a blowing agent.
 8. A method of making acuring agent according to claim 1 comprising the steps of (a) mixing atleast one of the mono- or diamine with the coumarin, optionally in thepresence of an polar solvent, (b) heating the reaction mixture to atemperature between room temperature and 70° C. until the monoamine ordiamine has completely reacted with the coumarin, (c) precipitating thereaction mixture into a nonsolvent for the coumarin amine reactionproduct, (d) filtering the precipitate from the liquid phase and (e)drying the coumarin amine reaction product.
 9. A method according toclaim 8 where the solvent is selected from methanol, ethanol, acetone,dimethylsulphoxide (DMSO), N,N-dimethylacetamide (DMA) andtetrahydro-furane (THF) or mixtures thereof.
 10. A method according toclaim 8 comprising the addition of a basic catalyst.
 11. A methodaccording to claim 9 wherein the basic catalyst is a heterocycliccompound having an amidine bonding.